Question:

When I want to merge records, I get a warning message in the Merge Preview:
"Please reach out to your Salesforce Administrator to merge the selected records. If you merge these records anyway the following field(s) will be populated with values from the master record: ..."

Answer:

This occurs when you have sufficient permissions to merge records of an object in general, but read-only access to specific fields. If the read-only access is a problem for the merge according to the field merge rules, this warning will appear. The listed fields are the fields that you have read-only access to.

Your Salesforce Administrator can merge the selected records according to the field merge rules. If you proceed to merge the records by yourself, the listed fields will take the values from the master record.